1. Field of the Invention
This invention relates to an apparatus and method for packaging semiconductor devices such as semiconductor integrated circuits in resin to protect them from their surroundings.
2. Description of the Prior Art showing a mold used in a transfer molding machine for resin-sealed semiconductor devices which is disclosed for example in Japanese Patent Application Laying-Open No. 68940/1983. A mold 1 attached to a hydraulic press comprises an upper mold section 2 and a lower mold section 3, which are separable from each other. The mold 1 is heated to 180.degree. C. by suitable heating means. Formed in the region of contact between the upper and lower mold sections 2 and 3 are a pot 4 for receiving a resin tablet 7, a plurality of cavities 6 communicating with said pot 4 through runners 5, and an exhaust port 12. Each cavity receives a semiconductor device such as a semiconductor integrated circuit.
To compress the resin tablet 7 disposed in the pot 4, there are provided a piston 8 and a hydraulic cylinder 9 which drives said piston 8. A seal member 10 is disposed around the outer periphery of the piston so that gas tightness can be maintained while the piston 8 is slid along the inner wall surface of the pot 4. A seal member is disposed in the region of contact between the upper and lower mold sections 2 and 3 to maintain gas tightness therebetween.
As shown in the figures, a vacuum pump 13 is provided for drawing off the air contained in the pot 4 and cavities 6. The resin tablet is formed of an epoxy resin powder compressed into a cylindrical form, the softening point thereof being 80.degree.-100.degree. C.
Referring to FIGS. 1 and 2, the known operation or steps involved in packaging semiconductor devices in resin will now be described.
First, the mold 1 comprising the upper and lower mold sections 2 and 3 is attached to the hydraulic press, and the mold 1 is heated to 170.degree.-180.degree. C. Thereafter, the upper and lower mold sections 2 and 3 are separated from each other, and semiconductor devices are put in the cavities 6. Further, the resin tablet 7, heated to 80.degree.-100.degree. C. in advance, is put in the pot 4. The upper and lower mold sections 2 and 3 are then clamped together.
The vacuum pump 13 is then operated to draw off the air contained in the pot 4 and cavities 6 After the pot 4 and cavities 6 have been substantially evacuated, the piston 8 is lifted to compress the resin tablet 7. The resin tablet 7 has had its temperature raised due to heat conduction from the mold 1 and its viscosity is consequently lowered. The resin tablet 7 is pressed by the piston 8, with the result that the resin is fed to the cavities 6 through the runners 5. Therefore, the semiconductor device disposed in each cavity 6 is entirely enclosed in resin. The resin will be cured in 2-3 minutes. Thereafter, the mold 1 is opened to take out the moldings. Necessary treatments are applied to the moldings thus taken out whereby the products are completed.
However, the conventional apparatus shown in FIGS. 1 and 2 and the method described above entail the following problems.
Heretofore, in consideration of the efficiency of use of resin, the inner diameter of the pot 4 has been made close to the outer diameter of the resin tablet 7. In other words, the ratio of the inner diameter of the pot 4 to the outer diameter of the resin tablet 7 is about 1:1. The resin tablet 7 contains air, moisture, low-boiling organic substances and the like. As a result, when a vacuum is created in the pot 4 by the vacuum pump 13, such gases as air in the resin tablet 7 expand and the resin tablet 7 expands radially Since the inner diameter of the pot 4 is approximately equal to the outer diameter of the resin tablet 7, as described above, the radial expansion of the resin tablet 7 causes the resin tablet 7 to intimately contact the inner wall surface of the pot 4. As a result, the air between said intimately contacted region and the piston 8 is confined as it is cut off from escape.
If the evacuating operation by the vacuum pump 13 is continued in this condition, the resin tablet 7 is pulled up in its entirety until it abuts against the upper mold section 2, as shown in FIG. 2, with part of the resin flowing into the runners 5. When the runners 5 are thus blocked by the resin, it is no longer possible to draw off the air contained in the pot 4, nor is it possible to extract the air contained in the resin tablet 7 even if the vacuum pump 13 is operated. After all, with the apparatus shown in FIGS. 1 and 2, it follows that with much air remaining in the pot 4 and in the resin tablet 7, a portion of the air remains mixed in the resin as the piston 8 is operated to press the resin tablet 7. Therefore, a large amount of air is trapped in each of the semiconductor devices molded in the cavities 6, leading to a decrease in the reliability of the semiconductor devices.
More particularly, the marks of gas bubbles in the form of recesses remain on the surfaces of the moldings, leading to a poor appearance and lead wire breaks Further, if such molding having a semiconductor device molded therein is placed in a wet environment, moisture which penetrates into the molding dissolves the low-boiling organic substances remaining therein, so that the penetrating water itself turns acidic or alkaline. As a result, this causes the problem of the penetrating water corroding the aluminum wiring formed on the chip of the semiconductor device.